A Passive Optical Network (PON) is a point-to-multipoint network architecture that brings optical fiber cabling and signals all or most of the way to the end user. A PON comprises an OLT at a service providers Central Office (CO) and a number of ONUs near end users. More than one ONU may be connected to the same OLT by means of passive or in some cases, active, intermediate elements. For example, up to 32 ONUs may be connected to an OLT. PONs are called passive because optical transmission requires no power or active electronic components. In other words, other than at the central office and subscriber/user endpoints, there are typically no active electronics within an access network. An access network is that part of a communications network which connects subscribers to their immediate service provider. It is contrasted with the core network. The core network is the central part of the communications network that provides various services to subscribers who are connected by the access network. Downstream signals, i.e. from the OLT to the ONU, are broadcasted to all users sharing a single fiber. Upstream signals are signals transmitted from the ONU to the OLT.
An ONU is a device that converts incoming downstream optical signals transmitted from the OLT via a wireline fiber optical communications channel, referred to as an optical link, into electrical signals. These electrical signals are then sent to individual subscribers over an optical fiber network, in order to provide telecommunications services. An ONU comprises an optical transceiver. The ONU may also be called an Optical Network Terminal (ONT), which is a special case of an ONU that serves a single subscriber.
As mentioned above, the ONU is used in combination with the OLT. The OLT performs conversion between the electrical signals used by the service provider's equipment and the fiber optic signals used by the PON. The OLT coordinates the multiplexing between the ONUs at or near the various subscriber locations.
PON system energy conservation has been embraced by the International Telecommunication Union (ITU) as a necessary goal. One target of the ITU is to explore potential solutions to improve energy conservation through reduced power consumption within optical access networks. These solutions may have impact on improving the equipment performance and service longevity in battery-powered operation, as well as on energy conservation and CO2 emission in general.
The term power refers to the ability to do work, while the term energy is a measure of the amount of work done, that is, power time. Therefore, energy may be saved or conserved, while power may be reduced. Throughout the industry, the term power is commonly used to refer to both. It is to be understood in the following description that terms such as power saving imply power reduction for a period of time, with the purpose of energy saving.
Wireless communication is a fast growing segment of the communications industry. Wireless cellular systems have experienced exponential growth over the last decade. Many new applications are emerging from research ideas to concrete systems. This explosive growth of wireless systems coupled with the proliferation of devices such as smart phones indicate a bright future for wireless networks.
A current power saving mode conserves energy by implementing sleep cycles to completely power off the ONU optical transceiver at certain times. Thus, at any given time, the ONU may be in so-called sleep mode or so-called awake mode, and it may use a wake-up timer to change between the modes. In order to support ONU wake-up, sleep cycles are provisioned by the OLT, typically in the order of 10-100 milliseconds. Only when the sleep cycle expires, does a sleeping ONU have the chance to wake up to receive downstream traffic. During a sleep period, the OLT may buffer downstream traffic destined to the ONU. When there is no downstream traffic to a sleeping ONU, the ONU still has to be awakened when the wake-up timer expires, i.e. when the sleep interval expires. Frequently turning the optical transceiver on and off contributes to extra power consumption. When there is downstream traffic destined to an ONU, and the ONU is still in sleep mode, the OLT has to buffer the traffic until the wake-up timer expires. The necessary buffer memory adds cost and power consumption to the OLT.
Another power saving mode is power shedding, in which non-essential services are switched off when the ONU goes into battery operation mode after e.g. an alternating current (AC) power failure. Power shedding has the target to reduce the demand for battery power and prolong the survival time of the ONU, while still maintaining lifeline Plain Old Telephone Service (POTS). The current definition of power shedding mode still maintains the optical transceiver on, while just powering down other selected elements in the ONU. However, one of the main power consumption contributors in the ONU is the optical transceiver module. Additional energy may be saved by combining power shedding with sleep mode.